arXiv subjects

J. S. Wu

Publications and source records attributed to J. S. Wu.

4 recordsLinked to original sources

arXiv2019

Electric field filamentation and higher harmonic generation in a very high frequency capacitive discharges

The effects of the discharge voltage on the formation and nature of electric field transients in a symmetric, collisionless, very high frequency, capacitively coupled plasma are studied using a self-consistent particle-in-cell (PIC) simulation code. At a driving frequency of 60 MHz and 5 mTorr of argon gas pressure, the discharge voltage is varied from 10V to 150 V for a fixed discharge gap. It is observed that an increase in the discharge voltage causes filamentation in the electric field transients and to create multiple higher harmonics in the bulk plasma. Correspondingly, higher harmonics, up to 7th harmonic, in the discharge current are also observed. The power in the higher harmonics increases with a rise in the discharge voltage. The plasma density continues to increase with the discharge voltage but in a non-linear manner, whereas, the bulk electron temperature decreases. Meanwhile, the electron energy distribution function (EEDF) evolves from a Maxwellian at lower discharge voltages to a bi-Maxwellian at higher discharge voltages.

physics.plasm-ph↗

arXiv2014

Infrared nano-spectroscopy and imaging of collective superfluid excitations in conventional and high-temperature superconductors

We investigate near-field infrared spectroscopy and superfluid polariton imaging experiments on conventional and unconventional superconductors. Our modeling shows that near-field spectroscopy can measure the magnitude of the superconducting energy gap in Bardeen-Cooper-Schrieffer-like superconductors with nanoscale spatial resolution. We demonstrate how the same technique can measure the c-axis plasma frequency, and thus the c-axis superfluid density, of layered unconventional superconductors with a similar spatial resolution. Our modeling also shows that near-field techniques can image superfluid surface mode interference patterns near physical and electronic boundaries. We describe how these images can be used to extract the collective mode dispersion of anisotropic superconductors with sub-diffractional spatial resolution.

cond-mat.supr-con↗

arXiv2004

Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem

A new phasing algorithm has been used to determine the phases of diffuse elastic X-ray scattering from a non-periodic array of gold balls of 50 nm diameter. Two-dimensional real-space images, showing the charge-density distribution of the balls, have been reconstructed at 50 nm resolution from transmission diffraction patterns recorded at 550 eV energy. The reconstructed image fits well with scanning electron microscope (SEM) image of the same sample. The algorithm, which uses only the density modification portion of the SIR2002 program, is compared with the results obtained via the Gerchberg-Saxton-Fienup HIO algorithm. In this way the relationship between density modification in crystallography and the HiO algorithm used in signal and image processing is elucidated.

physics.optics↗

arXiv2001

Bremsstrahlung of Flavor-Degenerate Pairs by Neutrinos in the Nuclear Field

Neutrino Bremsstrahlung of flavor-degenerate pairs in the field of a nucleus is of potential importance for neutrino astrophysics and is representative of a class of processes connecting leptonic electroweak sectors to real or virtual photons. We focus on first generation flavor production by both electron and muon neutrinos and present Standard Model cross sections and distributions for lead and iron nuclei. The results (of order 10-41 cm2 for electron neutrino - lead collisions at 100 MeV) have been fitted to empirical formulae that can be used to estimate backgrounds to neutrino detection experiments and flux normalizations. A compact form of the matrix element obtained by analytic reduction is used to explain the distributions. The V-A limits of the cross sections are shown to agree with published work from the pre-neutral current era. Event signatures and the possible roles of these processes in stellar and laboratory neutrino physics are discussed. Cross sections are compared with those for neutrino-electron scattering for neutrino spectra corresponding to typical supernovae temperatures.

astro-ph↗